1. Field
The present disclosure generally relates to optical networks. More specifically, the present disclosure relates to a multi-chip module (MCM) that includes integrated circuits that communicate via an optical network using tunable optical light sources, such as tunable-wavelength lasers.
2. Related Art
Wavelength division multiplexing (WDM), which allows a single optical link to carry multiple channels, can provide: very high bit-rates, very high bandwidth densities and very low power consumption. As a consequence, researchers are investigating the use of WDM to facilitate inter-chip communication. For example, in one proposed architecture chips (which are sometimes referred to as ‘sites’) in an array (which is sometimes referred to as an MCM or a ‘macrochip’) are coupled together by an optical network that includes optical interconnects (such as silicon optical waveguides).
In order to use photonic technology in interconnect applications, an efficient design is needed for the optical network. In particular, the optical network typically needs to provide: a high total peak bandwidth; a high bandwidth for each logical connection between any two sites in the array; low arbitration and connection setup overheads; low power consumption; and bandwidth reconfigurability.
A variety of network topologies having different characteristics and contention scenarios have been proposed to address these challenges in interconnect applications. One existing network topology, a static WDM point-to-point optical network, is shown in FIG. 1. In this network topology, an array of integrated circuits or chips 0-3 (which are each located at a ‘site’ in the array) are coupled by silicon optical waveguides using two carrier wavelengths (represented by the solid and dotted arrows). Note that the optical network in FIG. 1 is a fully connected point-to-point optical network. In particular, each site has a dedicated channel to every other site. Channels to all the sites in a column of the array (which are conveyed by different carrier wavelengths output by non-tunable light sources) may be multiplexed using WDM onto a single waveguide that runs from the source site and visits each site in the column, where a wavelength-selective ‘drop filter’ redirects one of the multiplexed wavelengths to a destination site (in this case, the drop filters in row 1 pick off the first carrier wavelength, and the drop filters in row 2 pick off the second carrier wavelength, so the carrier wavelength is used for routing). As illustrated by the bold line, in FIG. 1 chip 0 communicates with chips 1 and 3.
A key property of this optical network is the lack of arbitration overhead, which allows low minimum latency and high peak utilization for uniform traffic patterns. Furthermore, this optical network uses no switching elements, which results in low optical power loss in the optical waveguides. However, the bandwidth in the optical waveguides is statically allocated, which constrains the available bandwidth between any two sites. For example, in a macrochip that includes 64 chips arranged in an 8×8 array, with a peak system bandwidth of 20 TB/s, a total transmit bandwidth of 320 GB/s and a total receive bandwidth of 320 GB/s for each site, the bandwidth between any two sites is 5 GB/s, because each site has 64 outgoing optical waveguides so that each optical waveguide only has 1/64th of the total site bandwidth. This constraint can lead to low performance for workloads that heavily stress a subset of the optical waveguides.
Other proposed network topologies have attempted to address this problem at the cost of: additional power consumption (such as that associated with switches), optical signal loss, increased area, constraints on the total transmit and receive bandwidths, constraints on the optical waveguide density, latency associated with setting up switches, and/or arbitration overhead associated with shared resources (which can be a performance bottleneck for workloads consisting of short messages). To date, the tradeoffs between the improved site-to-site bandwidth and the costs in these other approaches do not successfully address the challenges in implementing optical networks in interconnect applications.
Hence, what is needed is an MCM with an optical network that does not suffer from the above-described problems.